Access channel structure for wireless communication system

ABSTRACT

A technique for efficient implementation of pilot signals on a reverse link in a wireless communication system. An access channel is defined for the reverse link such that within each frame, or epoch, a portion is dedicated to sending only pilot symbols. Another portion of the frame is reserved for sending mostly data symbols; however, within this second portion of the frame, additional pilot symbols are interleaved among the data symbols. The pilot symbol or preamble portion of the access channel frame allows for efficient acquisition of the access signal at the base station, while providing a timing reference for determining the effects of multipath fading. In particular, a pilot correlation filter provides a phase estimate from the pilot symbols in the preamble portion, which is then used to decode the data symbols in the payload portion. An access acquisition portion of the receiver uses the phase estimates provided by the pilot correlation filter to process the output of a data symbol correlation filter. The additional pilot symbols embedded in the payload portion are used in a cross product operation to further resolve the effects of multipath fading.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.09/766,875, filed Jan. 19, 2001, which claims the benefit of U.S.Provisional Application No. 60/181,071, filed on Feb. 8, 2000. Theentire teachings of the above applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of wireless digitalcommunications and more particularly to a technique for encoding accesschannel signals.

The increasing use of wireless telephones and personal computers haslead to a corresponding demand for advanced wireless communicationservices which were once thought only to be meant for use in specializedapplications. In particular, wireless voice communication first becamewidely available at low cost through the cellular telephone network. Thesame has also become true for distributed computer networks, whereby lowcost, high speed access to data networks is now available to the publicthrough Internet Service Providers (ISPs). As a result of the widespreadavailability of both technologies, the general population nowincreasingly wishes to be able to access the Internet using portablecomputers and Personal Digital Assistants (PDAs) over wireless links.

The most recent generation of wireless communication technologies makesuse of digital modulation techniques in order to allow multiple users toshare access to the available frequency spectrum. These techniquespurportedly increase system capacity for a radio channel of a givenavailable radio bandwidth. The technique which has emerged as mostpopular within the United States is a type of Code Division MultipleAccess (CDMA). With CDMA, each transmitted radio signal is first encodedwith a pseudorandom (PN) code sequence at the transmitter. Each receiverincludes equipment that performs a PN decoding function. The propertiesof the PN codes are such that signals encoded with different codesequences or even with different code phases can be separated from oneanother at the receiver. The CDMA codes thus permit signals to betransmitted on the same frequency and at the same time. Because PN codesin and of themselves do not provide perfect separation of the channels,certain systems have added an additional layer of coding, and/or usemodified PN codes. These additional codes, referred to as orthogonalcodes, and/or modified PN codes encode the user signals so that they aremathematically exclusive in order to further reduce interference betweenchannels.

In order for the CDMA code properties to hold true at the receiver,certain other design considerations must be taken into account. One suchconsideration involves the signals traveling in a reverse linkdirection, that is, from a field unit back to the central base station.In particular, the orthogonal properties of the codes are mathematicallyoptimized for a situation where individual signals arrive at thereceiver with approximately the same power level. If they do not,interference between the individual signals which arrive at the basestation increases. Precise control over the level of each signaltransmitted on the reverse link is thus critical.

More particularly, most CDMA systems are structured such that theforward link channels, that is, the channels carrying information fromthe base station towards the field unit, are different from the reversechannels. The forward link typically consists of three types of logicalchannels known as the pilot, paging, and traffic channels. The pilotchannel provides the field unit with timing and phase referenceinformation. Specifically, the pilot channel contains a sequence of databits that permits the field unit to synchronize its PN decoding functionwith the PN coding used in the base station. The pilot channel is,therefore, typically transmitted continuously by the base station tofacilitate the field units demodulation of the other forward linkchannels.

The paging channel is used to inform the field unit of additionalinformation needed to communicate. Such information is typicallymanagement information which informs the field unit of which trafficchannels it may use, for example. Other types of paging messages areused to communicate system parameters, access parameters, neighbor listsand other information needed for the field unit to manage itscommunication in such a way that it does not interfere with other fieldunits transmissions.

The forward traffic channels are used to transmit user data and/or voicesignaling information from the base station to the field unit.

On the reverse link, there are typically at least two types of logicalchannels, including an access channel and traffic channels. The accesschannel is used by the field unit to send a message to request access totraffic channels when it has data to communicate to the base station.The field unit thus uses the access channel to make requests forconnection originations and to respond to paging messages. The trafficchannels on the reverse link serve the same purpose as the trafficchannels on the forward link, namely, to transmit user data and/ordigitized voice payload information.

Pilot channels are not typically used on the reverse link. There areperhaps several reasons for this. For example, the most widely deployedCDMA systems, such as the IS-95 compatible system as specified by theTelecommunications Industry Association (TIA), use asynchronous reverselink traffic channels. It is typically thought that the overheadassociated with allowing each field unit to transmit on its owndedicated pilot channel is not necessary. It is also thought that theoverhead associated with decoding and detecting a large number of pilotchannels back at the base station would not justify any perceivedincrease in performance.

SUMMARY OF THE INVENTION

In general, pilot signals are advantageous since they provide forsynchronous communication. If the communications on the reverse linktraffic channels can be synchronized among various field units,parameters can be better optimized for each link individually. It wouldtherefore be advantageous to make pilot signals available for use on thereverse link.

Furthermore, the use of pilot channels on the reverse link would assistin combating effects due to multipath fading. Especially in urbanenvironments where many tall buildings and other surfaces may reflectradio signals, it is common for not just one version of each transmittedsignal to arrive at a receiver. Rather, different versions of aparticular transmitted signal, each associated with a particular delay,may be actually received. Having additional synchronization timinginformation available at the base station can help properly decodereverse link signals which have experienced a multipath fade.

The present invention is a technique for efficient implementation ofpilot signals on a reverse link in a wireless communication systemencompassing a base station which services a large number of fieldunits. According to one aspect of the invention, an access channel isdefined for the reverse link such that within each frame or epoch, apreamble portion of the frame is dedicated to sending only pilotsymbols. Another portion of each access channel frame, called thepayload portion, is then reserved for sending data symbols. In thispayload portion of the frame, additional pilot symbols are interleavedamong the data symbols.

In the preferred embodiment, the pilot symbols are inserted atpredictable, regular intervals among the data symbols.

The preamble portion of the access channel frame allows for efficientacquisition of the access signal at the base station, and provides atiming reference for separating the data and pilot symbols in thepayload portion, as well as a timing reference for, optionally, dealingwith the effects of multipath fading. This is accomplished by feedingthe preamble portion to a pilot correlation filter. The pilotcorrelation filter provides a phase estimate from the pilot symbols inthe preamble portion, which is then used to decode the data symbols inthe payload portion.

An access acquisition portion of the receiver then uses these phaseestimates provided by the pilot correlation filter to process the outputof a data symbol correlation filter.

The additional pilot symbols embedded in the payload portion arepreferably used in a cross product modulator to further undo the effectsof multipath fading.

The preamble portion of the frame may be defined by Barker sequences,which further assist with properly aligning the timing.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a block diagram of the system which uses embedded pilot symbolassisted coherent demodulation according to the invention.

FIG. 2 is a detailed view of the format of data framing used on theaccess channel.

FIG. 3 is a high level diagram of the pilot symbol assisted demodulationprocess.

FIG. 4 is a more detailed view of the pilot symbol assisted coherentdemodulators.

FIG. 5 is a still more detailed view of an access acquisition portion ofthe coherent demodulator.

FIG. 6 is a more detailed view of a data detection portion of thecoherent demodulator.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Turning attention to the drawings, FIG. 1 is a generalized diagramshowing a wireless data communication system 10 that makes use of anaccess channel having embedded pilot symbols in order to effectuatecoherent demodulation. The system 10 consists of a base station 12 and afield unit 20. The base station 12 is typically associated with apredetermined geographic region 14 in which wireless communicationservice is to be provided.

The base station 12 contains several components, including a radiotransmitter 15, receiver 16, and interface 17. The interface 17 providesa data gateway between the base station 12 and a data network 18 such asthe Internet, a private network, a telephone network, or other datanetwork.

The field unit 20 consists of a corresponding receiver 21, transmitter22, and interface 23. The interface 23 permits the field unit 20 toprovide data signals to and receive data signals from computingequipment 24 such as a laptop computer, Personal Digital Assistant(PDA), or other computing equipment. The interface 23 may be a PCMCIAbus, USB port, or other standard computer interface.

The base station 12 communicates with the field unit 20 by exchangingradio signals over various radio channels. The present invention is ofparticular advantage in a system 10 which uses Code Division MultipleAccess (CDMA) modulation to define the channels. In the specificembodiment discussed herein, it is therefore understood that a specificpseudorandom (PN) code (which may or may not be augmented withorthogonal codes) is used to define each of the various logical channelson a given radio carrier frequency.

The forward link 30 consists of various types of logical channels,including at least a pilot channel 31, a paging channel 32, and one ormore traffic channels 33. The forward link 30 is responsible forforwarding data signals from the base station 12 towards the field unit20.

The pilot channel 31 contains typically no baseband information, butrather a stream of bits that are used to permit the field unit 20 tosynchronize to the signals sent in the other forward link logicalchannels such as the paging channel 32 and traffic channel 33.

The paging channel 32 is used to transmit messages from the base station12 to the field unit 20 that control various aspects of communication,but most importantly, control assignment of various traffic channels 33for use by each field unit 20.

The forward traffic channels 33 are used to transmit data voice or othersignaling messages from the base 12 towards the field unit 20.

Signals are also carried from the field unit 20 towards the base station12 over a reverse link 40. The reverse link 40 contains several logicalchannel types including at least an access channel 41, a synchronization(sync) channel 42, and one or more traffic channels 43.

For the reverse link 40, the access channel 41 is used by the field unitto communication with the base station 12 during periods of time whenthe field unit 20 does not have a traffic channel 43 already assigned.For example, the field unit 20 typically uses the access channel 41 tooriginate request for calls as well as to respond to messages sent to iton the paging channel 32.

The sync channel 42 on the reverse link may assist in or with thetraffic channels 43 to permit the field unit 20 to efficiently send datato the base 12 using synchronous modulation techniques.

The present invention relates to the formatting and use of the reverselink access channel 41. Specifically, the invention uses an accesschannel 41 that contains within it certain formatting such as certainsymbols used to convey pilot signal information.

The access channel 41 signal format is shown in more detail in FIG. 2.An epoch or frame 50 consists of a preamble portion 51 and payloadportion 52. The preamble 51 is further defined as a series of symbolsincluding a pilot block 53 and Barker code block 54. Multiple pilotblocks 53 and Barker code blocks 54 make up the preamble 51; in theillustrated preferred embodiment, a pilot block 53 and Barker block 54are repeated four times in each frame 50. The Barker blocks 54 assist inallowing the receiver to determine where the start of a frame 50 is.

Each pilot block 53 consists of a number of repeated pilot symbols. Inthe preferred embodiment, 48 pilot symbols are repeated in each pilotblock 53. The pilot blocks 53 are used to assist with timing receptionand decoding of the information symbols which make up the access channel41.

The second portion of each frame 50 is the payload portion 52. Thepayload portion 52 includes a data portion consisting of the informationto be sent from the field unit 20 to the base 12. As shown in FIG. 2,pilot symbols 53 are inserted in the data portion of the payload 52. Apilot symbol, for example, may be inserted every eight payload symbols.As will be discussed in greater detail later, these pilot symbolsembedded in the payload portion 52 further assist with the coherentdemodulation process of the information contained in the data portion.

The pilot symbols 53 typically consist of a series of positive data bitsonly. Therefore, they do not in and of themselves contain timinginformation.

The Barker blocks 54 may consist of predetermined patterns of bits, asshown in FIG. 2. Binary Phase Shift Keyed (BPSK) bit encoding may beused to indicate a Barker sequence consisting of three positive bitsfollowed by three negative bits, followed by a single positive bit, apair of negative bits, a positive bit, and then a negative bit. Thepositive logic Barker sequence +B may be alternately sent with thenegative of the Barker sequence −B to further assist in aligning thebeginning of each frame 50 at the receiver 16.

The use of multiple pilot blocks 53 and Barker blocks 54 permit anaveraging process to be performed in the acquisition of each accesschannel 41 is described further below.

FIG. 3 is a generalized block diagram of the portion of the receiver 16used by the base station 12 to demodulate the reverse link accesschannel 41. As shown, the access channel receiver consists of twofunctions including access acquisition 60 and data decoding 62. In apreferred embodiment, multiple data decoding blocks 62-1, 62-2, 62-3 maybe used as individual rake receiver portions, or receiver “fingers,”tuned to different timing delays.

In general, the preamble pilot symbols are first processed by the accessacquisition function 60. These provide generalized timing informationwhich is then fed to the data decoding function 62, along with thepayload portion 53 containing the data symbols and embedded pilotsymbols. Each of the individual fingers 62-1, 62-2, 62-N make use of thetiming information provided by the access acquisition function 60 toproperly decode the data in the access channel.

This receiver signal processing can now be understood more readily byreference to FIG. 4, which is a more detailed diagram of both the accessacquisition function 60 and data decoding function 62. In particular,the access acquisition function 60 is seen to include a PilotCorrelation Filter (PCF) 70 as well as an integration function 72. Aswill be discussed in more detail below, the PCF 70 is a matched digitalfilter having coefficients matched to provide an impulse response toinput preamble pilot signals.

The integration function 72 operates on successive outputs of the pilotcorrelation filter 70 to provide a smoothed estimate of timinginformation inherent in the pilot symbols.

The data decoding portions 62 each include a data matched filter 80, aselection function 82, a dot or “cross” product function 84, integrationfunctions 86, and delay 88. A summer 90 operates on the outputs of theindividual data decoders 62-1, 62-2, . . . , 62-n to provide an estimateof the payload data. Briefly, each of the data decoders 62 operates as asynchronous demodulator to provide an estimate of the data symbols for agiven respective possible multipath delay. Although three data decoders62 are shown in FIG. 4, it should be understood that a smaller number ofthem may be used depending upon the anticipated number of multipathdelays in the system 10.

FIG. 5 is a more detailed block diagram of the access acquisitionportion 60. This circuit includes the previously mentioned pilotcorrelation filter 70 in the form of a pair of pilot correlation matchedfilters (PCMFs) 700-1, 700-2, and a corresponding pair of vectorinfinite impulse response (IIR) filters 710-1 and 710-2. In addition,the integration function 72 is provided by the pair of magnitudesquaring circuits 720-1 and 720-2, a summer 722, and threshold detector724.

In operation, the access channel 41 signal is fed to the pilotcorrelation matched filter (PCMF) sections 700-1 and 700-2. The pair ofPCMFs 700 are used in a ping pong arrangement so that one of the PCMFsmay be operating on received data while the other PCMF is having itscoefficients loaded. In the preferred embodiment, the access channel isencoded using 32 PN code chips per transmitted symbol. At the receiver,8 samples are taken per chip (e.g., 8 times the chip rate of 1.2288megahertz (MHz)). The pilot correlation matched filter 700 must not onlybe matched to receive the pilot symbols, but also to the particularpseudorandom noise (PN) code used for encoding the access channel. Acontroller 730 is used to control the operation of the two portions ofthe access acquisition circuit 60, both the top half and bottom half, asillustrated.

Continuing with the discussion of the Pilot Correlation Filter 70, thevector IIR filter 710-1 receives the output of the PCMF 700-1 in theform of in-phase (I) and quadrature (Q) samples. As shown in the signaldiagram 750 next to the output of the PCMFs 700, the output tends to bea series of peaks spaced apart in time, with the peak spacing, dependingupon the multipath delays experienced on the reverse link. For example,a peak occurring at a first time T1 may be associated with the mostdirect signal path taken. A second peak may occur at a time T2associated with a portion of the signal which follows an alternate path.Finally, a third peak may be associated with a time T3 which follows yeta different path from the field unit 20 to the base 12. The series ofpeaks are output for each of the 48 symbols in the pilot burst. Thefunction of the vector IIR filter 710-1 is thus to average these pilotbursts to provide a more well defined set of peaks 760 which representsthe outputs of the PCMF 700-1 averaged over time. The averaging processimplemented by the vector IIR filter 710-1 may, for example, eliminate afalse peak, such as that occurring at time T4, which is attributable toa noise burst and not to an actual multipath signal portion.

The output 760 of the vector IIR filter 710 thus represents an estimateof where the true multipath peaks occur in the reverse link accesschannel 41.

Of ultimate interest is the signal level of the received pilot signal.To determine this level, the magnitude block 720-1 takes the magnitudeof the vector IIR output signal 760. The sum circuit 722 thus sums thesesignals as provided by each of the two ping pong branches 700. Athreshold detector 724 is then applied to the summed signal to providean output similar to the plot 770. The threshold detector is set at apredetermined amplitude TH so that an output appears as in plot 780.

The points at which the summed signal output crosses the threshold THindicate points at which rake fingers 62 will be assigned to processesthe signal. In particular, the peaks occurring at times T1, T2 and T3are examined, and each respective time is used and assigned to arespective data matched filter 80 and the corresponding finger 62. Theseprovide an estimate of possible phases from the pilot symbols which isin turn used in the data decoding process.

FIG. 6 illustrates how the data detection process of the three rakefingers 62. Each finger 62 is identical. An exemplary rake finger 62-1consists of a corresponding Data Correlation Matched Filter (DCMF) 80-1,a peak sample detector 81-1, a switch 82-1, a vector IIR filter 83-1,complex conjugate function 85-1, and dot product circuit 84-1.

In operation, the access channel signal is first fed to the DataCorrelation Matched Filter (DCMF) 80-1. This filter 80-1 is loaded withcoefficients at a specific phase delay of the PN sequence. In thisinstance, the phase delay loaded is that data associated with the timeT1 indicated from the output of the access acquisition block 60.

The output of data correlation matched filter 80-1 will consist of asignal having a localized peak. As shown in the diagram next to the peaksample detector 81-1, the peak sample detector 81-1 selects apredetermined number of samples around this peak for further processing.

These peak values are then fed to the switch 82-1. The switch 82-1,under the operation of the data decoder controller 790, alternatelysteers the peak detected signal, depending upon whether it containspilot symbols or pilot plus data symbols. The decoder controller 790 maybe synchronized with a start of frame indication as determined by thereceived Barter symbols in the preamble portion, and therefore knows theposition of pilot symbols in the payload portion. Thus, while receivingthe payload or data portion 52 of the access channel frame 50, thesignal will be steered to the lower leg 88-1, in the case of receiving apilot symbol, or in the case of receiving a data symbol, will be steeredto the upper leg 89-1.

The pilot symbols of the payload portion 52 are processed in a mannersimilar to the pilot symbol processing in the preamble portion 51. Thatis, they are processed by a vector IIR filter 83-1 to provide anaveraged estimate of an estimate signal value [p]e^(j). The complexconjugate of this pilot estimate is then determined by the complexconjugate circuit 85-1.

Data symbols steered to the upper leg 89-1 provide a data estimatesignal x_(n)e^(je).

The two estimate signals, data and pilot are then fed to the multiplier84-1 to provide a cross product of the pilot symbols with the datasymbols. This causes the phase terms of the complex signal to cancelmore or less. That is, the phase estimate (theta) should beapproximately equal to the measured phase theta of the pilot symbols.The output thus represents the pilot channel energy |p|⁻x_(n). Given apilot symbol normalized value of 1, the data is therefore recovered.

Returning to FIG. 4, the reader will recall that this is the output ofonly one rake finger 62-1. Each rake finger output is, therefore, thenfed through the integrators 86, 87, additional dot product circuits 89,and delays 88-1, to the summer 90 to provide a final estimate of thedata, X.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method for processing access channel signals in a digital wirelesscommunication system comprising the steps of: receiving an accesschannel frame having a preamble portion containing pilot symbols encodedthroughout the preamble portion and a payload portion having datasymbols; obtaining a pilot symbol phase estimate from the pilot symbolsin the preamble portion; and using the pilot symbol phase estimate tosynchronize detection of the data symbols.
 2. A method as defined inclaim 1 wherein the payload portion contains pilot symbols interleavedwith the data symbols.
 3. A method as defined in claim 2 furthercomprising the step of: obtaining a data symbol estimate from the datasymbols.
 4. A method as defined in claim 3 further comprising the stepof: performing a cross product operation of the pilot symbols containedin the payload portion and the data symbol estimate.
 5. A method asdefined in claim 2 further comprising the steps of: performing a crossproduct operation of the pilot symbols contained in the payload portionand the data symbols.
 6. A method as defined in claim 2 wherein thepilot symbols are interspersed at regular intervals in the payloadportion.
 7. A method as defined in claim 1 further comprising the stepof: detecting the pilot symbols with a pilot correlation matched filterhaving a transfer function matched to the pilot symbols.
 8. A method asdefined in claim 1 further comprising the step of: detecting the datasymbols with a data correlation matched filter having a transfercharacteristic matched to the data symbols.
 9. A method as defined inclaim 1 further comprising the steps of: receiving a payload portionsequence of pilot symbols and data symbols; separating the payloadportion sequence into pilot symbols and data symbols usingsynchronization information derived from the pilot symbols in thepreamble portion; and comparing the separated pilot symbols and datasymbols to detect information received.
 10. A method as defined in claim8 further comprising the step of: performing a dot product of theseparated pilot symbols and data symbols.
 11. A method as defined in 1further comprising the steps of: feeding the preamble portion to a pilotcorrelation matched filter; and comparing an output of the correlationmatched filter to a peak detector.
 12. A method as defined in claim 11further comprising the steps of: detecting a time position of aplurality of peaks in an output of the peak detector; and setting aplurality of rake receivers to each of the detected peaks.
 13. A methodas defined in claim 1 further comprising the step of: encodingalternating blocks of pilot symbols and predetermined code sequencesthroughout the preamble portion of the access channel frame.
 14. Amethod as defined in claim 13 wherein the predetermined code sequencesare Barker code sequences.
 15. An apparatus for processing accesschannel signals in a digital wireless communication system, comprising:a pilot correlation filter configured to: (a) receive an access channelframe having a preamble portion containing pilot symbols encodedthroughout the preamble portion and a payload portion having datasymbols, and (b) obtain a pilot symbol estimate from the pilot symbolsin the preamble portion; and a data symbol correlator configured to usethe pilot symbol phase estimate to synchronize detection of the datasymbols.
 16. An apparatus as defined in claim 15 wherein the data symbolcorrelator is further configured to obtain a data symbol estimate fromthe data symbols.
 17. An apparatus as defined in claim 15 wherein thepayload portion contains one or more pilot symbols.
 18. An apparatus asdefined in claim 17 further comprising: a data decoder functionconfigured to perform a cross product operation of the pilot symbolscontained in the payload portion and the data symbols.
 19. An apparatusas defined in claim 17 further comprising: a data decoder functionconfigured to perform a cross product operation of the pilot symbolscontained in the payload portion and the data symbol estimate.
 20. Anapparatus as defined in claim 17 further comprising: a data correlationmatched filter having a transfer characteristic matched to the datasymbols in the payload portion wherein the data correlation matchedfilter is configured to detect the data symbols in the payload portion.21. An apparatus as defined in claim 15 further comprising: a pilotcorrelation matched filter having a transfer function matched to thepilot symbols wherein the pilot correlation matched filter is configuredto detect the pilot symbols in the preamble portion.